This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Quantitative Measurement of affinity between antibodies and antigens is of importance to investigate the molecular recognition specificity of antibodies. This becomes a highly interesting area driven by the extensive efforts in pharma industry to develop new drug and vaccine candidates that are large molecules. Our group developed PLIMSTEX (Protein Ligand Interaction by Mass Spectrometry, Titration and H/D Exchange) to study the affinity and conformational change that occurs upon ligand binding to proteins. Employing PLIMSTEX to an antibody-antigen interaction system will provide a novel analytic methodology for the measurement of the binding affinity of antibody-antigen interaction, which is complementary to ELISA and SPR.